Seismic sensor

ABSTRACT

A seismic sensor contains a geophone ( 4 ) disposed within a housing ( 12 ). The sensor is also provided with locking means ( 14, 19, 21 ) that can either lock the geophone  4  relative to the housing, or can allow the geophone to rotate within the housing. 
     The geophone ( 4 ) is enclosed in a casing ( 1 ) which is so constructed that the overall mass distribution of the casing and geophone is uneven. The casing and geophone assembly thus adopts a preferred orientation when the sensor is unlocked. 
     A liquid can be disposed within the housing, so that the casing ( 1 ) and geophone ( 4 ) can float when the sensor is unlocked.

The present invention relates to a seismic sensor, in particular to aseismic sensor that incorporates a geophone.

A geophone is a well-known seismic sensor (or seismic receiver) that isin widespread use in the field of seismic surveying. Typically, an arrayof geophones will be distributed around a source of seismic energy.Energy emitted from the seismic source is reflected by geologicalstructures within the earth, and the reflected energy is received at thegeophones. From an analysis of the energy reflected to each particulargeophone it is possible to derive information about the geologicalstructure of the earth.

A geophone incorporates a moving coil. Seismic energy incident on thegeophone induces vibrations of the coil, and an electrical output signalis derived from the vibrations of the coil. In order to ensure accurateoperation of the geophone the moving coil must be mounted with its axisvertical, or at least within ±10° of the vertical.

One type of prior art seismic sensor comprises a geophone that ispackaged in a housing. A spike is provided on the exterior of thehousing, and this spike extends in a direction substantially parallel tothe axis of the moving coil. Such a seismic sensor is used by pushingthe spike into the ground, to secure the sensor in position. Thisensures good coupling between the geophone and the ground, and if thespike is driven hard into firm ground the axis of the geophone isunlikely to move.

One disadvantage of this known seismic sensor is that, as noted above,it must be disposed such that the axis of the moving coil of thegeophone is within ±10° of the vertical. This means that after eachgeophone has been planted in the ground, it must be checked to ensurethat the axis of the moving coil is within 10° from the vertical, andthis is time-consuming where a large array of sensors is being used. Ifthe orientation of the geophones is not checked, and it should happenthat some of them are mounted with the axis of the moving coil at morethan 10° from the vertical axis, then the quality of the obtainedseismic data will be degraded. A further disadvantage of this prior artseismic sensor is that it is very difficult to deploy automatically,since the processes of planting the sensor in the earth and checking itsorientation is difficult to automate.

An alternative prior art seismic sensor that comprises a geophone hasthe geophone moveably mounted in a housing. The geophone is mounted ongimbals that incorporate low-friction ball bearings. Because thegeophone is mounted on gimbals, it can orientate itself such that theaxis of the moving coil is vertical regardless of the orientation of thehousing.

Although this prior art seismic sensor solves the problem of mountingthe sensor such that the axis of the coil is substantially vertical, itis not without its own disadvantages. In particular, external noisesources can generate unwanted oscillations of the geophone, and thiswill produce noise in the output signal from the geophone and so degradethe quality of the seismic data obtained. It is possible to introduce aviscous liquid into the housing to dampen oscillations of the geophoneinduced by external noise, but this does not provide a complete solutionsince it will not prevent oscillation of the geophone. A furtherdisadvantage of this type of seismic sensor is that they are expensive,owing to the need to provide the complex gimbal mechanism.

The present invention provides a seismic sensor comprising: a housing; ageophone rotatably mounted within the housing; and a locking means forreleasably preventing movement of the geophone relative to the housing.

A seismic sensor according to the present invention has two states:locked and unlocked. In the unlocked state, the geophone can move, forexample rotate, within the housing, in order to orientate itself withthe axis of its coil substantially vertical. Once the geophone iscorrectly orientated, the sensor can be locked. In its locked state,movement of the geophone relative to the housing is prevented. Since thegeophone is locked relative to the housing, a source of external noisewill not cause unwanted oscillations of the geophone, so that the sensorcan be used to obtain high quality seismic data.

In a preferred embodiment of the invention, the locking means comprisinga locking material disposed within the housing, the locking materialbeing solid at the normal operating temperature of the sensor. In orderto unlock the sensor, it is necessary only to heat the locking materialto its melting point; once the locking material melts, the geophone willbe able to rotate within the housing. Once the geophone is correctlyoriented, the sensor is locked simply by allowing the locking materialto cool below its melting point.

In an alternative preferred embodiment, the locking means is moveablebetween a first position in which it exerts a locking force on thegeophone so as to prevent movement of the geophone relative to thehousing and a second position. The sensor is locked or unlocked bymoving the locking means into the first position or the second positionrespectively.

Other preferred features of the present invention are set out in thedependent claims, to which attention is directed.

Preferred embodiments of the present invention will now be described byway of illustrative example with reference to the accompanying figuresin which:

FIG. 1(a) shows a casing for a geophone for use in a seismic sensoraccording to the invention;

FIG. 1(b) is a cross-sectional view through the casing of FIG. 1(a);

FIGS. 2, 3 and 4 show stages in the assembly of the casing shown in FIG.1(a);

FIG. 5 is a perspective partial view of a housing of a seismic sensoraccording to a first embodiment of the invention;

FIG. 6(a) is a plan view of the housing of FIG. 5;

FIG. 6(b) is a sectional view of the housing of FIG. 5;

FIG. 7 is a perspective view of a seismic sensor according to a secondembodiment of the present invention in the course of assembly;

FIG. 8 is a perspective view of a part of the housing of a seismicsensor according to the second embodiment of the invention;

FIG. 9 is a perspective view of a seismic sensor according to the secondembodiment of the present invention at a later stage of assembly;

FIGS. 10 and 11 are perspective views of a seismic sensor according tothe second embodiment of the present invention at a yet later stage ofassembly; and

FIG. 12 is a perspective view of further part of the housing of aseismic sensor according to the second embodiment of the invention.

FIG. 1(a) shows a casing 1 for a geophone used in a seismic sensor ofthe present invention. As will be seen in FIGS. 1(b) and 2, a geophone 4is disposed within the casing 1, and is oriented such that the axis ofthe moving coil of the geophone is substantially parallel to the axisY—Y shown in FIGS. 1(a) and 2. As explained above, in use the casing 1is preferably oriented within the sensor such that the axis Y—Y issubstantially vertical so that good quality seismic data is obtained.The construction of the casing 1 will be described with reference toFIGS. 1(a) to 4.

The casing comprises first and second electrode portions 2, 3. Each ofthese is electrically connected to one of the output contacts 2′, 3′ ofthe geophone 4.

The casing 1 further comprises first and second casing portions 5, 6. Itis undesirable for the geophone 4 to vibrate within the casing, sincethis would degrade the seismic data and could also result in physicaldamage to the geophone. The interior of the casing portions 5, 6therefore preferably define a recess that is so dimensioned and soshaped that the geophone 4 fits snugly into the recess and is preventedfrom vibrating within the casing.

In the embodiment shown in FIGS. 3 and 4 one of the casing portions 5 isprovided with a protruding portion 7 that fits into a recess 8 providedin the other casing portion, to locate the two casing portions 5, 6together. The invention is not, however, limited to this method oflocating the casing portions together. The casing portions 5, 6 arepreferably secured to one another by a suitable adhesive, since thisprovides a low-cost assembly. Other conventional methods of securing thecasing portions 5, 6 to one another can alternatively be used, such asscrews, or nuts and bolts.

The casing 1 further comprises a weight 9. This is provided at thebottom of the casing 1 (here “top” and “bottom” refer to the preferredorientation of the casing when the sensor is in operation). The weight 9is sufficiently heavy so that the centre of gravity of the geophone andcasing assembly is below the geometric centre of the casing when thecasing is oriented with the axis Y—Y vertical. In FIG. 1(b) the weight 9has the form of a flat disc, but the shape of the weight is not limitedto this particular shape.

The casing is completed by an end casing portion 32. This is secured tothe casing portions 5,6 in any suitable way, for example in one of waysdescribed above for securing the casing portions 5,6 to one another.

FIG. 1(b) is a cross-section of the assembled casing 1. It will be seenthat parts of the interior of the casing are empty (that is air-filled),and these reduce the overall density of the casing and geophone.

The outer surfaces of the electrode portions 2, 3, and the casingportions 5, 6, 32 are parts of the surface of a sphere. The casing 1 isthus substantially spherical.

A protruding circumferential ridge 10 is provided on the outer surfaceof the casing 1, for reasons that will be discussed below.

The components of the casing can be made of any suitable material. Inprinciple the electrodes 2, 3 may be formed of a plastics material thatis provided with a conductive coating. However, this can lead toproblems with the conductive coating peeling off and the electrodes 2,3are preferably metallic.

The materials of the casing portions 5, 6, 32 must be chosen so that theelectrodes 2, 3 are electrically insulated from one another. The casingportions 5, 6, 32 can be made from any electrically insulating materialwith suitable structural properties such as, for example, a plasticsmaterial. The weight 9 can be made from a dense material such as, forexample, steel or lead.

FIGS. 5 to 6(b) show a seismic sensor according to a first embodiment ofthe present invention. This seismic sensor comprises a geophone enclosedwith a casing of the type shown in FIG. 1, and this casing is disposedwithin a housing 11. The housing 11 is constructed from two housingportions 11 a, 11 b. One of the housing portions 11 b is shown in FIGS.5 and 6(b).

The housing 11 comprises a recess 12 for receiving a geophone 4 that hasbeen enclosed in a casing 1 of the type shown in FIG. 1. The shape ofthe recess is complementary to the shape of the casing 1 enclosing thegeophone, although the size of the recess 12 is greater than the size ofthe casing 1.

The first housing portion 11 b is shown in FIG. 5 in perspective view,and in FIG. 6b in plan view. The recess 12 is defined partially in thefirst housing portion 11 b, and partially in the second housing portion11 a, which is generally similar in shape to the first housing portion11 b.

First and second housing electrodes 13, 14 are provided in the recess12. In the embodiment of FIGS. 5 to 6(b), these are provided on thefirst housing portion 11 b. In use, the casing 1 enclosing the geophoneis disposed within the recess 12 such that one of the casing electrodes2 makes contact with one of the housing electrodes 13 and the other ofthe casing electrodes 3 makes electrical contact with the other housingelectrode 14. The inner surfaces of the housing electrodes 13,14 areshaped to be complementary to the outer surfaces of the casingelectrodes 2,3 of the casing 1, so as to provide good electricalcontact. In this embodiment the casing electrodes 2,3 have asubstantially spherical outer surface, and the inner surface of each ofthe housing electrodes 13,14 is therefore also spherical.

One of the housing electrodes 13 is fixedly mounted within the recess12. However, the other housing electrode 14 is mounted in the recess 12such that it can be moved towards the centre of the recess or retractedfrom the centre of the recess, along the axis A—A indicated in FIG. 5.In this embodiment the movable mounting of the housing electrode 14 isachieved by providing the moveable housing electrode with a shaft 15that projects from the outer surface of the electrode and passes throughan aperture 16 in the wall 17 defining the recess 12 within the housingportion 11 b. A notch 18 is provided in the shaft 15 of the moveablerecess electrode. A locking member 19, which is pivotally mounted at oneend on a pivot 20, passes through the notch in the shaft 15 of themoveable housing electrode 14. By moving the end of the locking lever 19remote from the pivot 20 in the direction indicated by the arrow B, itis possible to move the shaft 15 along the axis A—A, towards the centreof the recess 12, thereby moving the movable housing electrode 14towards the fixed housing electrode 13. Moving the end of the lockinglever 19 remote from the pivot in the opposite direction to thatindicated by the arrow B will cause the movable housing electrode 14 tomove away from the fixed housing electrode 13.

The size of the recess 12 within the housing is made sufficiently largeso that, when the casing 1 is disposed within the recess 12, it ispossible to lock or unlock the casing 1 relative to the housing bymoving the moveable housing electrode 14 towards or away from the fixedhousing electrode 13. That is, when the moveable housing electrode 14 ismoved as far away from the fixed housing electrode 13 as possible, thedistance between the inner faces of the moveable and fixed housingelectrodes should be greater than the diameter of the casing 1, so thatthe casing 1 can rotate within the recess 12. Conversely, by moving themoveable electrode 14 towards the fixed electrode 13, the separationbetween the fixed and moving housing electrodes 13, 14 can be made equalto the diameter of the casing 1 so that the casing is gripped betweenthe fixed and moveable housing electrodes. The frictional forcegenerated between the housing electrodes 13, 14 and the casing 1 willprevent rotation of the casing within the recess 12. Thus, in thisembodiment electrical connection to the geophone is provided by thelocking means when the sensor is in the locked state.

Means for actuating the locking lever 19 are provided. In the embodimentshown in FIG. 5, the actuating means comprise a stepping motor 21. Thestepping motor is provided with two pairs of electrical leads 30 a,30 b;31 a,31 b that are connected to contacts (not shown) provided on theoutside of the housing portion 11 b so that an electrical current can besupplied to the stepping motor 21.

The locking lever 19 is biased in the direction indicated by the arrow Bin FIG. 5. The bias force tends to urge the moveable housing electrode14 towards the fixed housing electrode 13, so that the sensor is biasedtowards its locked state. In FIG. 5 the bias force is applied by a coilspring 33, one end of which is attached to the wall 17 defining therecess 12 and the other end of which is attached to the locking lever19. The spring 33 is in tension, and so exerts a force on the lockinglever that tends to bias the sensor into its locked state. The biasmeans is not, however, limited to the coil spring shown in FIG. 5, andany suitable means for biasing the sensor towards its locked state canbe used.

The stepping motor 21 has two coils, one connected to one pair of leads30 a,30 b and the other connected to the other pair of leads 31 a,31 b.To drive the stepping motor such that its armature moves in a directionopposite to the arrow B, a pulse signal or square wave signal is appliedto each coil, with the signal applied to the second coil having a phaseshift of 90° compared to the signal applied to the first coil. With eachpair of pulses that is applied the armature of the motor will move byone step in the direction opposite to the arrow B. To reverse thedirection of motion of the armature, the phase shift between the twopulses is changed to −90°, so that with each pair of pulses that isapplied the armature of the motor will move by one step in the directionof the arrow B.

In order to unlock the sensor, the stepping motor 21 is actuated toapply a force in the opposite direction to the arrow B to the end of thelocking lever 19 that is remote from the pivot 20. This is done byapplying pairs of pulses to the stepping motor 21 as described above.The result of applying such a force to the remote end of the lockinglever is that the locking lever 19 is caused to rotate about the pivot20 so as to move the moveable housing electrode 14 away from the fixedhousing electrode 13 thereby putting the sensor into the unlocked state.In order to lock the sensor, the armature of the motor is moved in thedirection of the arrow B, by changing the phase shift between thecurrent pulses applied to the two coils of the stepping motor, and thebias force applied to the locking lever ensures that the sensor isreturned to its locked state.

One advantage of using a stepping motor as the actuating means is thatthe armature of the motor will always move by the same distance for eachpair of pulses that is applied. Thus, the position of the armature canbe calculated from knowledge of the number of pairs of pulses applied tothe stepping motor, and it is not necessary to provide a detection meansto determine the position of the locking lever. A further advantage ofusing a stepping motor is that a stepping motor can be driven by adigital input signal.

The armature of the stepping motor 21 should remain fixed unless a pulseis applied to the motor. This means that, in principle, the bias meanscould be omitted and the stepping motor could be used to lock and unlockthe sensor.

Although a stepping motor is used to actuate the locking lever 19 in theembodiment of FIGS. 5, 6(a) and 6(b), the invention is not limited tothis. Any suitable actuating means can be used to actuate the lockinglever 19, such as, for example, a solenoid or a d.c. current motor. Ifactuating means other than a stepping motor are used, it might benecessary to provide detection means, such as a feed-back loop, todetect the amount of movement of the locking lever.

In the embodiment of FIGS. 5, 6(a) and 6(b) the actuating means (thestepping motor 21) is disposed within a chamber 28 defined within thehousing 11. This is preferable since it provides protection for theactuating means, but in principle, the locking lever could protrudethrough an aperture in the housing with the actuating means beingdisposed outside the housing.

In the embodiment of FIGS. 5, 6(a) and 6(b) the chamber 28 for theactuating means is provided predominantly within the first housingportion 11 b, but it would be possible for the chamber for the actuatingmeans to be provided substantially equally within the two housingportions.

The first housing portion 11 b is provided with raised protrusions 26,27. When the two housing portions are assembled, these protrusions fitinto complementary recesses provided on the second housing portion 11,to ensure correct alignment of the two housing portions. The two housingportions are preferably secured together in a way that allows the sensorto be dis-assembled to allow the components of the sensor to be servicedor replaced, and this can be done for example using one or more bolts44. In principle, however, the housing portions could be joined togetherusing an adhesive.

To assemble a sensor of this invention, the two housing portions 11 a,11 b are assembled with the casing 1, which contains the geophone 9,located within the recess 12 defined within the housing 11. The casingis shown in broken lines in FIG. 6(b), which is cross-section throughFIG. 6(a) long the line A—A.

In operation, the sensor is placed in its preferred location, and thehousing is secured in a particular orientation. The orientation of thehousing is not particularly critical, since it is possible to adjust theorientation of the geophone within the housing, by unlocking the sensor.If the sensor is intended for use on land a spike can be provided on theoutside of the housing to allow the sensor to be secured in a fixedorientation at a desired position; if provided, such a spike would alsoprovide good seismic coupling between the sensor and the ground. Thesensor is preferably locked during the process of positioning thesensor, to prevent damage to the geophone.

Once the housing of the sensor has been secured in position the sensoris then unlocked, to allow the casing to adopt, under the action ofgravity, an orientation in which the weight 9 is lowermost and the axisof the moving coil of the geophone 4 is vertical. Thus, a sensor of thepresent invention can easily be oriented so that the axis of the coil ofthe geophone is vertical, since the orientation of the geophone itselfis independent from the orientation of the housing. Moreover, since thesensor is unlocked using the actuating means 21, the sensor can beunlocked by a remote operator or even automatically.

When the geophone has been oriented correctly, the sensor is locked. Inits locked state, the casing 1 is gripped firmly between the moveableelectrode 14 and the fixed electrode 13, so that movement of the casingrelative to the housing is prevented. Locking the sensor in this wayprevents external noise from causing unwanted vibrations of the coil ofthe geophone.

Locking the sensor also establishes electrical contact between the fixedand moveable electrodes 13, 14 and the geophone, via the electrodeportions 2, 3 provided on the casing. As is shown in FIG. 5, the housingelectrodes 13, 14 are provided with electrical leads 23, 24 that passout of the recess 12 to contacts (not shown) provided outside thehousing 11, thus allowing the electrical signals produced by thegeophone to be connected to suitable monitoring equipment. Furthermore,when the sensor is locked, good seismic coupling is provided between thehousing and the casing 1, and hence to the geophone.

To make the process of orienting the geophone within the housing moreefficient and reliable to carry out, it is desirable to ensure that thefriction between the casing 1 and the housing 12 when the when thesensor is unlocked is as low as possible. In principle, friction couldbe reduced by, for example, providing the exterior of the casing 1and/or the interior of the housing 12 with a low friction coating or bypolishing the exterior of the casing and the interior of the housing. Ina preferred embodiment of the invention, however, friction between thecasing and the housing is reduced by providing a liquid in the recess 12so that the casing 1 can float when the sensor is unlocked. This can bedone, for example, by introducing a liquid into the recess 12 through anaperture (not shown) provided in the housing 11 for this purpose, afterthe housing has been assembled. A second aperture (also not shown) isprovided to allow air to be expelled from the recess during introductionof the liquid. The quantity of liquid introduced into the recess shouldnot be sufficient to completely fill the recess, and the liquidpreferably has a density that is greater than the overall density of thecasing 1 containing the geophone 4 so that the casing 1 can float in theliquid. It is preferable that the liquid used in not electricallyconductive and does not attack the components of the sensor. It is alsopreferable that the liquid will remain in the liquid state throughoutthe intended operating temperature range of the sensor.

For land applications, a suitable liquid is dibromomedhane (CH₂Br₂).This is liquid in the temperature range from −52° C. to +96° C., and hasa density of 2500 kg/m³. For marine applications, tetrabromoethane(C₂H₂Br₄) can also be used. This is a liquid in the temperature range of1° C. to 135° C. and has a density of 2,967 kg/m³.

In an embodiment in which a liquid is disposed in the recess 12, whenthe sensor is unlocked, by moving the moveable electrode 14 away fromthe fixed electrode 13 to release the casing 1, the casing 1 will floatin the liquid provided within the recess 12. When the casing floats inthe liquid there will be little or no contact between the casing and thehousing, so that friction between the casing and the housing will bevirtually eliminated. When the sensor is unlocked, the casing willquickly adopt an orientation in which the weight 9 is lowermost and theaxis Y—Y of the coil of the geophone is vertical.

In an embodiment where the recess 12 in the housing contains liquid, aseal 29 is preferably provided where the shaft 15 of the movable housingelectrode passes through the aperture 16 in the wall 17, to prevent theliquid from leaking out of the recess 12 while allowing low frictionmotion of the shaft 15 through the aperture 16.

It will be appreciated that excessive rotation of the casing 1 when thesensor is unlocked is undesirable. For example, it might be possible forone of the electrodes 13, 14 on the housing to short out the twoelectrodes 2, 3 on the casing. Alternatively, if the casing were torotate by 180°, polarity inversion would occur. In order to preventexcessive rotation of the casing, the casing electrodes 13, 14 are sodimensioned that, in conjunction with the ridge 10 provided on theexterior of the casing, they prevent excessive rotation of the casing 1.Alternatively, stops could be provided on the inside of the housing forlimiting the rotation of the casing.

A second embodiment of a sensor according to the present invention isillustrated in FIGS. 7 to 12. This embodiment is generally similar tothe embodiment of FIGS. 5, 6(a) and 6(b), and only the differencesbetween the embodiments are described below. Like components in the twoembodiments are denoted by like reference numerals.

FIG. 7 is a partial perspective view of a sensor according to the secondembodiment of the invention during the assembly of the sensor. Thesensor comprises a first housing portion 11 b that defines part of therecess 12 and also defines part of the chamber 28. The chamber 28 isseparated from the recess 12 by a wall 17 and, as in the firstembodiment, the locking lever and actuating means (not shown in FIG. 7)are disposed within the chamber 28. A casing 1, containing a geophone 4,is disposed in the recess 12 defined in the housing portion 11 b of thesensor. The housing portion 11 b is also provided with the pivot 20 forthe locking lever, and a mount 42 for the actuating means.

The housing portion 11 b shown in FIG. 7 is generally similar to thehousing portion 11 b of the first embodiment. However, in the secondembodiment of the invention, the housing is formed from three housingportions, rather than two housing portions as in the embodiment of FIGS.5-6(b).

A second of the housing portions that make up the housing 11 of a sensoraccording to the second embodiment of the invention is shown in FIG. 8.The second housing portion 11 c also defines part of the recess 11. FIG.9 shows the sensor during a later stage of assembly, after the recess 12has been closed by assembling the housing portions 11 b and 11 c.

Two apertures 34, 35 are provided in the wall 17 that defines the recess12 within the housing 11. In an embodiment in which a liquid is providedwithin the recess to allow the casing 1 to float when the sensor isunlocked, it is possible to introduce the liquid into the recess 12through one of the apertures 34, 35, with the other aperture acting as avent to allow air to be expelled from the recess during the introductionof the liquid. Once a sufficient quantity of liquid has been introducedinto the recess, the apertures 34, 35 are sealed.

In this embodiment the apertures 34, 35 are defined partially in onehousing portion 11 b and partially within the second housing portion 11c. It is, however, possible for the apertures to be defined entirelywithin one of the housing portions 11 b, 11 c. In principle, it is alsopossible for the apertures to be provided in one of the external wallsof the housing, rather than in the wall 17 that defines the recess 12within the housing.

In this embodiment the locking lever 19 is again biased so that thelocking lever tends to lock the casing 1 relative to the housing. FIG. 9illustrates one possible form of bias means, and this is a coil spring33, one end of which is attached to the locking lever 19 and the otherend of which is attached to the wall 17 defining the recess. The spring33 is in tension, and thus biases the sensor towards its lockedposition. Preferably, a number of possible attachment points for thespring are provided on both the locking lever 19 and the wall 17defining the recess, so that the distance between the two attachmentpoints of the spring 33 (that is, the point of attachment of the springto the locking lever and the point of attachment to the wall 17) can bechanged. This allows the bias force exerted by a particular spring to beadjusted, since the restoring force exerted by a given spring willdepend upon the length to which it is stretched.

In this embodiment of the invention, the housing portion 11 b isprovided with a recess 36 for holding a circuit board, such as a printedcircuit board. FIGS. 10 and 11 show, from different angles, the sensorafter a printed circuit board 37 has been disposed in the recess 36.During assembly of the sensor, electrical leads within the sensorhousing, such as the connections to the actuating means and theconnections from the fixed and moveable housing electrodes 13, 14 areconnected to the printed circuit board within the housing.

The housing portion 11 b is also provided with apertures 38, 39 in itsside walls, so that the printed circuit board 37 can extend to theoutside of the housing. Electrical connectors 40, 41 are attached to theprinted circuit board 37 at positions outside the housing, and thisfacilitates making electrical connections to the sensor.

In the embodiment shown in FIG. 10 the printed circuit board extendsacross the width of the housing, and an aperture 43 is provided withinthe printed circuit board to allow the locking lever to pass through theprinted circuit board 37. In principle, the printed circuit board couldpass through the housing 11 at only one point, so that it would not benecessary for the printed circuit board 37 to extend across the entirewidth of the housing 11 in this case, one of the apertures 38,39 in thehousing portion 11 b could be omitted.

The third portion 11 d of the housing of the sensor of the secondembodiment of the invention is shown in FIG. 12. The third housingportion 11 d closes the part of the housing that has not been closed bythe housing portion 11 c—that is, it closes the chamber 28. It can beseen that the second and third housing portions 11 c and 11 d of thesecond embodiment generally correspond to the housing portion 11 a ofthe first embodiment. In the second embodiment the chamber 28 issubstantially symmetrically disposed between the first and third housingportions 11 b, 11 d, but in principle it could be asymmetricallydisposed between the two housing portions.

The third housing portion 11 d also comprises a recess 36 for receivingthe printed circuit board 37, a mount 42 for the actuating means, andapertures 38,39 to allow the printed circuit board to pass outside thehousing of the sensor. As explained above with regard to the secondhousing portion 11 c, one of the apertures 38,39 could be omitted if theprinted circuit board 37 does not extend across the entire width of thehousing.

Assembly of the sensor shown in FIGS. 10 and 11 is completed bydisposing a suitable actuating means in the mount 42 in the housingportion 11 b. The actuating means is then electrically connected to theprinted circuit board 37, as are the connection 23 from the fixedhousing electrode and the connection (not shown) from the moveablehousing electrode. Finally, the third housing portion 11 d is attachedto the first and second housing portions 11 b, 11 c to close the chamber28 and so complete the housing 11.

Protrusions 26,27 and complementary recesses 26′,27′ are provided onopposing edges of the housing portions 11 b, 11 c, 11 d, to enable thehousing portions to be located together correctly. The housing portion11 c is preferably secured to the housing portion 11 b with a suitableadhesive. This enables a fluid-tight seal to be made at relatively lowcost. The third housing portion 11 d can also be secured to the housingportion 11 b by an adhesive. However, if it is desired to be able toservice or replace components within the housing, such as the spring 33,the actuating means, or the printed circuit board, it is possible tosecure the third housing portion 11 d to the housing portion 11 b in away that allows the two housing portions to be easily disassembled. Forexample, the third housing portion 11 d can be secured to the housingportion 11 b using screws or bolts.

The locking means of a seismic sensor of the present invention is notlimited to the locking means shown in FIGS. 5 to 12. Any locking meansthat can releasably lock the geophone relative to the housing can beused.

In another embodiment of the present invention (not illustrated), thelocking means is provided by a material within the recess 12 that issolid at the normal operating temperature of the sensor. In order tounlock the sensor, the locking material is heated to a temperature aboveits melting point. When the locking material melts, the casing 1 will bereleased, and will be able to rotate within the recess 12, so allowingthe geophone to orient itself relative to the housing. Once the geophoneis correctly oriented, the locking material is allowed to cool andsolidify, so that movement of the geophone within the housing isprevented.

The locking material can be any material that has a melting point thatis greater than the upper end of the desired temperature range ofoperation of the sensor. It is also, of course, preferred that thelocking material does not attack the materials of which the housing orcasing are made, and is not electrically conductive.

It is preferable that the melting point of the locking material is notsignificantly greater than the highest intended operating temperature ofthe sensor, since this will minimise the amount of heat required to meltthe locking material in order to unlock the sensor. Where the sensor isintended for use on land, it is usually desirable that the melting pointof the locking material is in the range of from 60° C. to 80° C. In thecase of a sensor intended for use in a marine environment, the maximumrequired operating temperature will be lower, and the melting point ofthe locking material is preferably greater than 25° C., for example inthe range 25° C. to 30° C.

For land applications, a wax or a similar material can be used as thelocking material. A wax having a low melting point, of around 25° C. to30° C., is suitable for a sensor intended for marine use.

In an embodiment in which the locking means is provided by a materialwithin the recess 12 that is solid at the normal operating temperatureof the sensor, the geophone is preferably disposed in a casing of thetype shown in FIG. 1, in order to provide protection for the geophone.The casing would be disposed within a housing generally similar to thehousing of the first or second embodiment, except that the means formoving the moveable electrode 14 of the first and second embodimentswould not be required. Instead, a heater for heating the lockingmaterial would be disposed within the housing. In this embodiment, theelectrical connections can be made to the geophone using low frictionmetallic contacts as in a conventional gimballed geophone.Alternatively, the electrical connections can be made using electricalwires of sufficient length, and a sufficiently small diameter, that theydo not restrict the rotation of the casing 1 within the housing.

In principle, the heater for heating the locking material could bedisposed on or within the casing 1 of the geophone rather than on thehousing 11. It is, however, preferable to place the heater within thehousing since this makes it easier to make electrical connections to theheater. Furthermore, the risk of the geophone being damaged byoverheating is also reduced.

In the embodiments of the invention described above the geophone isenclosed in a substantially spherical casing, and the recess 12 providedin the housing is also substantially spherical. The invention is not,however, not limited to a spherical casing, and other shapes could beused for the casing. The use of a spherical casing and a sphericalrecess in the housing are, however, preferable, since this allows thecasing to rotate freely around any axis with regard to the housing.

In principle, it would be possible not to provide the casing, and simplydispose a geophone within the recess. (If the casing were omitted,however, it would be necessary to ensure that the geophone had an unevendistribution of mass, for example by adding a bias weight, so that itwould adopt an orientation with the axis of its coil vertical when thesensor was unlocked.) Providing the casing is, however, preferable,since this provides greater physical protection for the geophone andthus increases the robustness and reliability of the sensor.

A further advantage of providing a casing is that the average density ofa commercially available moving coil geophone is around 4500 kg/m³. Thisis too heavy to float on most available liquids, except for mercury. Byusing a lightweight casing, the average overall density of the casingand geophone is reduced. A further reduction in the average overalldensity is obtained if the casing contains air spaces, as is the case ofthe casing of FIGS. 1(a) and 1(b). For a given geophone, the larger thecasing in which it is enclosed, the more air will be contained withinthe casing, and the lower will be the overall density of the casing andgeophone assembly. This means that if a liquid having a relatively highdensity, such as CH₂Br₂ (density 2500 kg/m³) is used, a given geophonewill require a smaller casing than if the liquid used has a relativelylow density, such as silicon oil (density: 960 kg/m³). For example, aparticular geophone might require a casing with an external diameter of47 mm when used with CH₂Br₂, but a casing with an external diameter of65 mm if used with silicon oil.

In the embodiment described with reference to FIGS. 5 to 12 above one ofthe housing electrodes is fixed. It would be possible for both housingelectrodes to be movably mounted in the housing, so that the actuatingmeans would cause both electrodes to move, in opposite directions.

What is claimed is:
 1. A seismic sensor comprising: a housing; ageophone rotatably mounted within the housing; and a locking means forreleasably preventing movement of the geophone relative to the housing,the locking means being movable between a first position in which itexerts a locking force on the geophone so as to prevent movement of thegeophone relative to the housing and a second position, and the lockingmeans providing an electrical connection to the geophone when thelocking means is in the first position.
 2. A seismic sensor as claimedin claim 1 and comprising bias means for biasing the locking meanstowards one of the first position and the second position.
 3. A seismicsensor as claimed in claim 1 wherein the geophone is mounted within acasing, and the locking means exerts a locking force on the casing inits first position.
 4. A Seismic sensor as claimed in claim 1 furthercomprising means for reducing friction between the geophone and thehousing, or between the casing and the housing, when the sensor isunlocked.
 5. A seismic sensor as claimed in claim 4 wherein the meansfor reducing fiction comprises a liquid disposed within the housingwhereby the geophone floats in the liquid when the sensor is unlocked.6. A seismic sensor as claimed in claim 5, wherein the liquid has adensity greater then the overall density of geophone, or than theoverall density of the casing and the geophone.
 7. A seismic sensor asclaimed in claim 1, further comprising stop means for limiting themovement of the geophone relative to the housing.
 8. A seismic sensor asclaimed in claim 1, wherein the mass distribution of the geophone, or ofa casing and the geophone, is not constant whereby the geophone adopts apreferred orientation relative to a vertical axis when the sensor isunlocked.